The dialkylaminoalkyl(meth)acrylates corresponding to formula (I):H2C═C(R′i)-C(O)—O-A-N(R′2)(R′3)  (I)in which:    R′i is a hydrogen atom or a methyl radical    A is a linear or branched. C1-C5 alkylene radical    R′2 and R′3, which may be identical or different, each represent a C1-C4 alkyl radical,are generally obtained by a transesterification reaction between an amino alcohol, of formula (II):HO-A-N(R′2)(R′3)  (II)in which A, R′2 and R′3 are as defined above, and a light alkyl(meth)acrylate of formula (III):CH2═C(R′i)-COOR′4  (III)in which R′i is as defined above and R′4 represents the methyl or ethyl radical.
The reaction is generally carried out in a stirred reactor in the presence of a transesterification catalyst and at least one polymerization inhibitor and the light alkyl(meth)acrylate/light alcohol R′4OH azeotropic mixture generated during transesterification is withdrawn continuously during the reaction.
The problems that arise during manufacture of the dialkylaminoalkyl(meth)acrylates, notably the formation of heavy byproducts resulting from Michael addition reactions between the compounds present in the reaction mixture, will now be described, for convenience, on the basis of the example of N,N-dimethylaminoethyl acrylate (designated ADAME hereinafter) corresponding to the following formula (Ia):H2C═CH—COOCH2CH2N(CH3)2  (Ia)obtained by transesterification reaction between a light alkyl acrylate and N,N-dimethylaminoethanol (DMAE).
The problems and the solution proposed by the invention are the same in the case when on the one hand a light alkyl methacrylate, or on the other hand amino alcohols other than DMAE, are used in the transesterification reaction.
The industrial process for manufacturing ADAME, such as that described for example in the applicant's patents EP 0960 0877 or FR 2 811 986, consists of a transesterification reaction between ethyl acrylate (EA) or methyl acrylate (MA) and N,N-dimethylaminoethanol (DMAE).
This reaction is generally catalyzed by tetraalkyl titanate such as tetraethyl titanate in solution in the DMAE in the case of the ADAME prepared starting from EA, and by a tin derivative (dibutyltin oxide (DBTO) or distannoxane) in the case of ADAME prepared starting from MA.
The reaction is generally carried out in the presence of a polymerization inhibitor and in the presence of air depleted to 8% oxygen (% by volume).
The light alcohol, methanol or ethanol, that forms during the reaction is distilled as it is formed, in the form of an MA/methanol or EA/ethanol azeotrope. This process can be carried out as a batch process or continuously, for example in a stirred reactor.
The crude reaction product generally contains the ADAME produced, unreacted light ester (EA or MA), light alcohol generated (ethanol or methanol), residual DMAE alcohol, heavy byproducts, the catalyst, and the polymerization inhibitors.
Regarding the side reactions leading to the formation of heavy byproducts during the manufacture of (meth)acrylic monomers, there is notably a reaction of Michael addition of a molecule containing a labile hydrogen atom (such as an alcohol) on the double bond of a (meth)acrylic compound.
For example, in the case of the manufacture of ADAME, the DMAE alcohol that has not yet reacted or the light alcohols formed (methanol or ethanol) add onto the double bond of the ADAME already formed or of the unreacted light acrylate (MA or EA), to form heavy byproducts of Michael addition [DMAE+ADAME] of formula:

or [DMAE+MA/EA] of formula:
R═CH3 or C2H5 
A Michael addition of the DMAE alcohol on oligomers of ADAME or of EA or of MA is also possible.
A characteristic of these heavy byproducts is that their boiling point is above the boiling points of the products used in the reaction and of the desired ADAME.
In the conventional process, generally the transesterification reaction is followed by various purification steps, generally by distillation, for final recovery of the purified ADAME.
One method consists for example of submitting the crude reaction product to a distillation (tailing to remove the catalyst and the heavy products) in order to separate the ADAME with the residual light products at the top of the distillation column, and the catalyst, the heavy byproducts, the polymerization inhibitors with a minor fraction of ADAME and of DMAE and traces of light compounds at the bottom of the distillation column.
The compounds distilling at the top of the tailing column are then separated in two other distillation columns in series (topping and final rectification) to recover pure ADAME at the top of the last column.
The bottom fraction from the tailing column can be sent to a film evaporator in order to recover and recycle the traces of light compounds. The bottom fraction from the evaporator is then generally discarded.
The presence of the heavy byproducts in this fraction that is discarded poses a treatment problem, as the heavy byproducts must be incinerated for disposal. There is also a problem of large losses of raw materials (notably DMAE) and of finished product (ADAME), which are present in this fraction in free form or in the form of Michael adducts.
To recover the upgradable products present in a residue from separation of the catalyst in a process for producing (meth)acrylic esters by transesterification, it was proposed in document U.S. Pat. No. 7,268,251 to treat the residue thermally according to a method comprising the following steps:                step 1: cracking the residue in the presence or absence of a catalyst at a temperature of 100-220° C. The light products from cracking are then recycled to one of the distillation columns of the purification train.        step 2: the residue from step 1 comprising the polymerization inhibitors, polymers and the catalyst, is submitted to a transesterification reaction at 80-150° C. in the presence of a heavy alcohol such as glycerol of 2-ethylhexanol in order to adjust the viscosity of this fraction and make the residue pumpable. The light products generated are recycled to the reaction.        
The process described in this document for recovering the upgradable materials contained in the residue of heavy byproducts is difficult to implement and has the following drawbacks:                the residue from the first step is very viscous and therefore difficult to transport by pump; that is why it must be modified by post-transesterification;        there is severe fouling of the cracking reactor, necessitating frequent cleaning with large decreases in heat exchange capacities;        cleaning is very difficult because the fouling materials adhere very strongly to the wall of the cracking reactor;        the light alcohols generated in step 2 of post-transesterification are recycled with considerable risk of contamination by the impurities present in the heavy alcohol used in this step;        the introduction of a heavy alcohol that is completely alien to the products naturally present in the ADAME facility imposes management constraints and presents risks of contamination.        
To tackle the problem of deposition of heavy compounds during the synthesis, purification or regeneration of (meth)acrylic monomers, document FR 2 876 374 proposes the use of phosphorus-containing antifouling agents; however, there is no question in this document of recovering the noble products from a heavy fraction generated during the production of (meth)acrylic esters by transesterification, leading to a fluid final residue.
In the process for synthesis of Ci-C4 alkyl(meth)acrylates described in document FR 2 901 272, the heavy byproducts are upgraded on the basis of a treatment of distillation/thermal cracking of a bottom product. This document relates to a synthesis by direct esterification in the presence of sulfuric acid, and the operation of thermal cracking also requires the presence of sulfuric acid.
Therefore there is still a need for a simplified process for recovery of the upgradable compounds contained in a residue of heavy (meth)acrylic byproducts.
One of the aims of the present invention is therefore to upgrade the noble products (starting compounds or finished product) potentially recoverable from the heavy fraction generated in a process for synthesis of (meth)acrylic esters by transesterification. This upgrading leads to improvement of the materials balance of the process and to reduction of the final amounts of residue to be incinerated, and consequently it represents an economic advantage.
The present invention relates to a treatment process for upgrading heavy fraction containing distillable or potentially distillable products after cracking, said process only requiring a moderate number of steps without fouling the equipment used and producing a final residue of low enough viscosity to be transported by pump an incinerated.
The process of the invention is particularly advantageous compared to the process described in U.S. Pat. No. 7,268,251 since it does not employ heavy alcohol that is alien to the process, thus avoiding the risk of contamination by recycling the compounds recovered.